A. Field of the Invention
The present invention relates to an emitter of infrared radiation in band III and to a composite allowing the emission of such infrared radiation.
B. Discussion of the Related Art
Emitters of infrared radiation in band II (wavelengths of the infrared radiation from 3 to 5 xcexcm) are known which can be mounted on a flying craft, for example a target airplane, in order to simulate the optical signature of certain types of aircraft particularly for the purpose of carrying out test firings for weapons, such as aircraft-destroying missiles.
The present invention relates to an emitter capable of emitting infrared radiation in band III (8 to 12 xcexcm in wavelength) and intended, in particular, to be used in the same type of application, in order to simulate the optical signature of other types of aircraft.
It is known that, for such applications (mounting on an aircraft flying at high speed, in order to simulate the optical signature of airplanes emitting in band III), the emitter must have particular characteristics, especially relating to:
the luminance, which must be of the order of 100 W/sr between wavelengths of 8 and 10 xcexcm;
the capability of withstanding the mechanical and climatic environmental stresses, which has to be compatible with the envisioned operating conditions (mounting on a flying craft able to fly at a speed close to Mach 1 and able to ascend to an altitude of 4000 m);
the overall size (the emissive surface area must remain less than a few hundred cm2); and
the cost, which must remain low.
Many solutions are conceivable, but none of them allows all the aforementioned conditions to be met. By way of illustration, mention may be made of:
a pyrotechnic solution using a powder tracer. The implementation of such a solution and, especially, the maintenance of stable combustion appear to be difficult, if not impossible, under the operating conditions envisioned. In addition, the luminance power capable of being emitted in band III seems to be insufficient;
a solution using a laser. This solution is prohibitive in terms of cost, weight, size and autonomy; and
Nernst lamps (bars of refractory material heated by resistance heating). These lamps are too fragile and the power emitted in band III is greatly insufficient for the applications envisioned.
Moreover, a device is known for simulating a signature in band III which is based on the emission of a highly emissive heated body. This known device comprises a metal dome which is heated by a propane burner. Said device makes it possible to attain a luminance of the order of 40 W/sr when it is mounted on a flying craft which flies at moderate speed (75 m/s).
However, this known device cannot be used for the applications envisioned in the present invention. This is because:
the luminance level obtained is insufficient (40 W/sr instead of 100 W/sr); and, in addition,
the cooling due to the aerodynamic flux at the high flight speed envisioned (280 m/s) would cause the temperature of the dome to drop and the luminance level to fall.
The object of the present invention is to remedy these drawbacks. It relates to a less-expensive and more compact emitter, making it possible to emit infrared radiation in band III, which has the aforementioned characteristics and which can be used in the applications indicated above.
For this purpose, according to the invention, said emitter of infrared radiation in band III is noteworthy in that it comprises:
an emission source comprising a composite which includes a standard and common metal, for example copper or nickel, on which a thin oxide layer, for example having a thickness of the order of 50 xcexcm, is deposited, said oxide having, in addition, an emissivity which is:
less than 0.2, at least for wavelengths of emitted radiation of less than 6 xcexcm; and
greater than 0.8 for wavelengths of between 8 xcexcm and 10 xcexcm; and
a heating device which can heat said composite so that it emits infrared radiation in band III.
Thus, in particular by virtue of the emissivity characteristics of said oxide (for example aluminum, magnesium or yttrium oxide), the emitter according to the invention is able to emit radiation in band III with high enough luminous energy for the applications envisioned. In addition, since the radiation emitted has a very low luminance in band I (1 xcexcm-1.5 xcexcm) and band II (3 xcexcm-5 xcexcm), the energy consumption is consequently reduced, thereby making it possible to optimize the overall energy yield of said emitter.
It should be noted that, by definition, the emissivity of a body is a dimensionless parameter, which expresses the ratio of the luminance emitted by this body to the maximum luminance of an ideal body called xe2x80x9cblack bodyxe2x80x9d. The value of this parameter varies between 0 and 1, depending on the material and on the wavelength.
Furthermore, by depositing the oxide on a metal, the cost, robustness, machining, heating and supply problems, which would exist if the oxide were to be used by itself, under the surface-area (150 cm2) and temperature (800xc2x0 C.) conditions envisioned here, are solved.
In addition, the use of a metal (for example irradiated platinum, nickel, copper or titanium), combined with said heating device, allows effective heating of said composite to a prescribed temperature of between 500xc2x0 C. and 1000xc2x0 C., preferably about 800xc2x0 C.
Preferably, said metal is hemispherical in shape and said oxide is deposited on the hemispherical external face of said metal.
Moreover, said heating device advantageously comprises means for controlling the heating temperature and it preferably heats by resistance heating. Other known heating modes may, of course, also be envisioned.
Furthermore, in one particular embodiment, the emitter according to the invention also includes:
a reflector allowing the infrared radiation emitted by the emission source to be directed in a predefined solid angle, thereby making it possible to increase the overall yield of the emitter; and/or
a casing which contains the emission source, so as to protect it from the outside, and which is provided with a window transparent to the infrared radiation emitted by said emission source, thereby making it possible in particular to isolate the emissive source from the external aerodynamic flux.
The present invention also relates to a composite comprising a metal and an oxide, for the emission of infrared radiation in band III, said composite and especially the oxide having the aforementioned properties.